Single-electron transistor with wrap-around gate

ABSTRACT

Transistors and methods of forming the same include forming a fin having an active layer between two sacrificial layers. A dummy gate is formed over the fin. Spacers are formed around the dummy gate. The dummy gate is etched away to form a gap over the fin. Material from the two sacrificial layers is etched away in the gap. A gate stack is formed around the active layer in the gap. Source and drain regions are formed in contact with the active layer.

BACKGROUND Technical Field

The present invention relates to semiconductor devices and, moreparticularly, to single-electron transistors.

Description of the Related Art

As conventional fabrication methods improve, and logic devices such astransistors become smaller, devices approach a limit of what can beachieved using standard complementary metal-oxide semiconductor (CMOS)processes. Single-electron transistors (SETs), which operate by quantumtunneling effects, can provide logic devices at even smaller scales thanCMOS.

SUMMARY

A method for forming a transistor includes forming a fin comprising anactive layer between two sacrificial layers. A dummy gate is formed overthe fin. Spacers are formed around the dummy gate. The dummy gate isetched away to form a gap over the fin. Material from the twosacrificial layers is etched away in the gap. A gate stack is formedaround the active layer in the gap. Source and drain regions are formedin contact with the active layer.

A method for forming a transistor includes forming a fin comprising anactive layer between two sacrificial layers. A dummy gate is formed overthe fin. Spacers are formed around the dummy gate. The dummy gate isetched to form a gap over the fin. Material from the two sacrificiallayers is etched in the gap, leaving the active layer suspended at bothends in the gap. A gate stack is formed around the active layer in thegap. Forming the gate stack includes conformally forming a gatedielectric layer on surfaces in the gap and forming a full wrap-aroundgate around the active layer. The active layer is etched after formingthe gate stack to form a quantum dot. Exposed surfaces of the quantumdot are oxidized to form tunnel barriers. Source and drain regions areformed in contact with the tunnel barriers.

A transistor includes a tunnel barrier structure. A gate stack wrapsfully around the tunnel barrier structure. A conductive source and drainregion, in contact with opposite sides of the tunnel barrier structure,pass through openings in respective sides of the gate stack.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a circuit diagram of a single-electron transistor inaccordance with the present principles;

FIG. 2 is a cross-sectional diagram of a step in forming asingle-electron transistor in accordance with the present principles;

FIG. 3 is a top down diagram of a step in forming a single-electrontransistor in accordance with the present principles;

FIG. 4 is a cross-sectional diagram of a step in forming asingle-electron transistor along a first cut in accordance with thepresent principles;

FIG. 5 is a cross-sectional diagram of a step in forming asingle-electron transistor along a second cut in accordance with thepresent principles;

FIG. 6 is a cross-sectional diagram of a step in forming asingle-electron transistor along the first cut in accordance with thepresent principles;

FIG. 7 is a cross-sectional diagram of a step in forming asingle-electron transistor along the second cut in accordance with thepresent principles;

FIG. 8 is a cross-sectional diagram of a step in forming asingle-electron transistor along the first cut in accordance with thepresent principles;

FIG. 9 is a cross-sectional diagram of a step in forming asingle-electron transistor along the second cut in accordance with thepresent principles;

FIG. 10 is a cross-sectional diagram of a step in forming asingle-electron transistor along the first cut in accordance with thepresent principles;

FIG. 11 is a cross-sectional diagram of a step in forming asingle-electron transistor along the second cut in accordance with thepresent principles;

FIG. 12 is a cross-sectional diagram of a step in forming asingle-electron transistor along the first cut in accordance with thepresent principles;

FIG. 13 is a cross-sectional diagram of a step in forming asingle-electron transistor along the first cut in accordance with thepresent principles;

FIG. 14 is a cross-sectional diagram of a step in forming asingle-electron transistor along the first cut in accordance with thepresent principles;

FIG. 15 is a cross-sectional diagram of a step in forming asingle-electron transistor along the first cut in accordance with thepresent principles; and

FIG. 16 is a block/flow diagram of a method of forming a single-electrontransistor in accordance with the present principles.

DETAILED DESCRIPTION

Embodiments of the present invention provide single-electron transistors(SETs) that operate at high temperature and have a wrap-around gate,with a process flow that is fully compatible with planar fabricationprocesses. The present embodiments employ a quantum dot that issurrounded by gate material in one cross-section and has a very smalldiameter.

Referring now to FIG. 1, a schematic of an SET 100 is shown. The SET 100includes a quantum dot 102 (also known as an “island”) between a sourceterminal 104 and drain terminal 106. A gate 108 is capacitively coupledto the quantum dot 102. The SET 100 has a blocking state and aconducting state, depending on a gate voltage applied to the gate 108.

The SET 100 operates by quantum tunneling. A drain voltage is applied tothe drain terminal 106 (while the source terminal 104 is grounded). Inthe blocking state, electrons at the source terminal 104 cannot tunnelonto the quantum dot 102, as there are no appropriate energy levels forit there. Applying the gate voltage to the gate 108, however, causes theenergy levels of the quantum dot 102 to change, putting at least oneenergy level within reach of the electron. The electron tunnels to thequantum dot 102 and subsequently tunnels to the drain terminal 106 toform a current, such that the SET 100 is in a conducting state when thegate voltage is applied.

The operation of the SET 100 is characterized by the capacitances of thedifferent components. In particular, a total capacitance (C_(total)) isthe sum of the gate capacitance (between the quantum dot 102 and thegate 108), the drain capacitance (between the quantum dot 102 and thedrain terminal 106), and the source capacitance (between the quantum dot102 and the source terminal 104). It should be noted that the quantumdot 102 is selected to have low self-capacitance. To operate an SET 100at high temperature, the size (namely, C_(total)) of the quantum dot 102should be very small, such that the charging energy of an electron(q²/2C_(total)) is larger than 20KT, where q is the charge of anelectron, K is the Boltzmann constant, and T is the temperature.

Referring now to FIG. 2, a step in fabricating an SET is shown. Asemiconductor substrate 202 is formed from an appropriate semiconductormaterial. It should also be understood that the substrate may instead beformed as a semiconductor-on-insulator substrate, instead of a bulksemiconductor. The semiconductor substrate 202 may be formed from asilicon-containing material. Illustrative examples of silicon-containingmaterials suitable for the semiconductor substrate 202 may include, butare not limited to, silicon, silicon germanium, silicon germaniumcarbide, silicon carbide, polysilicon, epitaxial silicon, amorphoussilicon, and multilayers thereof. Although silicon is predominately usedin wafer fabrication, alternative semiconductor materials can beemployed, such as, but not limited to, germanium, gallium nitride,cadmium telluride, and zinc sellenide.

Further alternatives for the semiconductor layer 202 include III-Vcompound semiconductors. The term “III-V compound semiconductor” denotesa semiconductor material that includes at least one element from GroupIII of the Periodic Table of Elements (International Union of Pure andApplied Chemistry Group 13) and at least one element from Group V of thePeriodic Table of Elements (International Union of Pure and AppliedChemistry Group 15). Typically, the III-V compound semiconductors arebinary, ternary or quaternary alloys including III/V elements. Examplesof III-V compound semiconductors that can be used in the presentembodiments include, but are not limited to alloys of gallium arsenic,aluminum arsenic, indium gallium arsenic, indium aluminum arsenic,indium aluminum arsenic antimony, indium aluminum arsenic phosphorus,indium gallium arsenic phosphorus and combinations thereof.

A first layer of sacrificial material 204 is formed on the substrate202. It is specifically contemplated that the first layer of sacrificialmaterial 204 may be formed from silicon germanium, though it should beunderstood that any appropriate material may be used instead. An activelayer 206 is formed on the first layer of sacrificial material 204. Itis specifically contemplated that the active layer 206 may be formedfrom doped silicon. A second layer of sacrificial material 208 is formedon the active layer 206. It is specifically contemplated that the secondlayer of sacrificial material 208 may be formed from the same materialas the first layer of sacrificial material 204 (e.g., silicongermanium), but any appropriate material may be used instead.

It is specifically contemplated that the active layer 206 may be doped.In one particular embodiment, an n-type dopant may be used. As usedherein, “n-type” refers to the addition of impurities that contributesfree electrons to an intrinsic semiconductor. In a silicon containingsubstrate examples of n-type dopants, i.e., impurities, include but arenot limited to antimony, arsenic and phosphorous. The dopant may beformed in situ with the active layer 206 or may, alternatively, beimplanted by bombardment. The dopant may have an exemplary concentrationbetween zero and about 5×10²¹ atoms/cm³.

Referring now to FIG. 3, a top-down view of a step in fabricating an SETis shown. One or more fins 302 are formed by etching the stack ofsacrificial layers 204/208 and the active layer 206. The fins 302 may beformed by any appropriate process including, e.g., photolithography,whereby regions of the top surface are covered with a photoresist andsubsequently exposed to a light that defines the fin pattern. Eitherexposed or unexposed regions of the resist are removed, depending on theresist chemistry, and the remaining portions (either photoresist orhardmask, for example, silicon nitride) are used as a mask for asubsequent etch. The etch may be any appropriate anisotropic etch suchas, e.g., reactive ion etching (RIE). RIE is a form of plasma etching inwhich during etching the surface to be etched is placed on aradio-frequency powered electrode. Moreover, during RIE the surface tobe etched takes on a potential that accelerates the etching speciesextracted from plasma toward the surface, in which the chemical etchingreaction is taking place in the direction normal to the surface. Otherexamples of anisotropic etching that can be used at this point includeion beam etching, plasma etching or laser ablation. Alternatively, thefin structure 302 can be formed by spacer imaging transfer processes.

The figure shows two different cross sections, A and B. Cross section Ashows a cut that runs parallel to the length of the fin 302, while crosssection B shows a cut that runs perpendicular to the length of the fin302. Subsequent figures will show one cross section or the other.

Referring now to FIG. 4, a cross-sectional view of a step in fabricatingan SET is shown along cut A. A dummy gate 406 and spacer 404 are formedover the fin 302. Although it is specifically contemplated that thedummy gate 406 may be formed from amorphous silicon or poly-silicon, anyappropriate material may be used instead. The spacer 404 is formed onthe sidewalls of the dummy gate 406 and may be formed from siliconnitride, but it should be understood that any appropriate hardmaskmaterial may be used instead. Exemplary alternative hardmask materialsinclude silicon oxides, silicon oxynitrides, silicon carbides, siliconcarbonitrides, and spin-on dielectrics such as silsequioxanes,siloxanes, and boron phosphate silicate glass.

An inter-layer dielectric 402 is deposited over the fin 302 and thedummy gate 406 using, for example, CVD or any other appropriatedeposition process. The inter-layer dielectric material is then polisheddown to the level of the top of the dummy gate 406 using, e.g., chemicalmechanical planarization (CMP). CMP is performed using, e.g., a chemicalor granular slurry and mechanical force to gradually remove upper layersof the device. The slurry may be formulated to be unable to dissolve,for example, the dummy gate material, resulting in the CMP process'sinability to proceed any farther than that layer.

Referring now to FIG. 5, a cross-sectional view of a step in fabricatingan SET is shown along cut B. As can be seen, the dummy gate 406 coversthe fin 302, which has a narrow profile.

Referring now to FIG. 6, a cross-sectional view of a step in fabricatingan SET is shown along cut A. The dummy gate 406 is selectively etchedaway using, e.g., a wet or dry etch, leaving behind the inter-layerdielectric 402, the spacers 404, and the fin 302. As used herein, theterm “etch selectivity” in reference to a material removal processdenotes that the rate of material removal for a first material isgreater than the rate of removal for at least another material of thestructure to which the material removal process is being applied.

Referring now to FIG. 7, a cross-sectional view of a step in fabricatingan SET is shown along cut B. As can be seen, fin 302 is fully exposedwithin the gap 602.

Referring now to FIG. 8, a cross-sectional view of a step in fabricatingan SET is shown along cut A. An isotropic etch is used to remove exposedsacrificial material from the fin 302. The isotropic etch has etchselectivity with the remaining active material in the fin 302. As aresult, the sacrificial layers 802 are removed around the active layer804 in the fin 302, with some amount of lateral etching occurring.

Referring now to FIG. 9, a cross-sectional view of a step in fabricatingan SET is shown along cut B. As can be seen, the active layer 804 issuspended in the middle of the gap 602.

Referring now to FIG. 10, a cross-sectional view of a step infabricating an SET is shown along cut A. A layer of gate dielectric 1002is formed on the surfaces of the gap 602. It is specificallycontemplated that the gate dielectric 1002 may be a high-k dielectric,but any appropriate insulating material may be used instead. A high-kdielectric is a material having a dielectric constant k that is greaterthan that of silicon dioxide. Exemplary high-k dielectric materialsinclude hafnium oxides, zirconium oxides, lanthanum oxides, aluminumoxides, titanium oxides, strontium titanium oxides, lanthanum aluminumoxides, yttrium oxides, hafnium oxynitrides, zirconium oxynitrides,lanthanum oxynitrides, aluminum oxynitrides, titanium oxynitrides,strontium titanium oxynitrides, lanthanum aluminum oxynitrides, yttriumoxynitrides, and multilayers thereof.

The remainder of the gap 602 is then filled with a gate 1004. The 1004may be formed from any appropriate gate material. In one exemplaryembodiment, the gate 1004 be formed from metal, but it should beunderstood that alternative gate materials may include polysilicongermanium, polysilicon/metal silicide, germanium, silicon germanium,silicon germanium carbide, metal silicides, metallic nitrides, metalssuch as tungsten, iridium, rhenium, ruthenium, titanium, tantalum,hafnium, molybdenum, niobium, nickel, or aluminum, or any otherappropriate conductive material.

Referring now to FIG. 11, a cross-sectional view of a step infabricating an SET is shown along cut B. The gate 1004 completelysurrounds the suspended active layer 804 in this plane, with a layer ofgate dielectric 1002 between the active layer 804 and the gate 1004.

Referring now to FIG. 12, a cross-sectional view of a step infabricating an SET is shown along cut A. The inter-layer dielectric 402is removed, along with any portions of the sacrificial layers 802 andactive layer 804 that are not covered by the spacers 404, gatedielectric 1002, and gate 1004. The material may be removed using ananisotropic etch such as RIE, leaving remnant sacrificial layers 1202and the trimmed active layer 1204.

Referring now to FIG. 13, a cross-sectional view of a step infabricating an SET is shown along cut A. A lateral wet etch is used toremove the remnant sacrificial layers 1202 and to further reduce thesize of the trimmed active layer 1204, producing quantum dot 1302. It isspecifically contemplated that the quantum dot 1302 may have anexemplary diameter of less than about 6 nm. The quantum dot 1302 isshown as being recessed from the surfaces of the gate dielectric 1002,but it should be understood that the quantum dot 1302 may have anyrelationship to the surface of the gate dielectric, including beingflush therewith.

Referring now to FIG. 14, a cross-sectional view of a step infabricating an SET is shown along cut A. Tunnel barriers 1402 is formedon the quantum dot 1302 by oxidizing the surface of the quantum dot1302. It is specifically contemplated that this will form a silicondioxide, although it should be understood that other forms of insulatingbarrier may be formed by other processes and in conjunction with otheractive layer materials.

Referring now to FIG. 15, a cross-sectional view of a step infabricating an SET is shown along cut A. Source and drain regions 1502are formed on either side of the tunnel barriers 1402 and quantum dot1302. The source and drain regions 1502 may be any appropriateconductive material and may be deposited using, e.g., CVD or epitaxialgrowth from the semiconductor substrate 202. Electrical contacts (notshown) may then be formed to the source/drain regions 1502 and the gate1004 to provide connectivity to the SET.

If a semiconductor material is used for the source and drain regions1502, such as silicon, it is specifically contemplated that the sourceand drain regions 1502 may be doped or silicided to provide increasedconductivity. Doping for the source and drain regions 1502 may beprovided in situ during deposition or growth or may, alternatively, beimplanted. A silicide may be formed by depositing a refractory metal,such as nickel or titanium, onto the surface of a silicon-containingmaterial. Following deposition, the structure is then subjected to anannealing step such as, but not limited to, rapid thermal annealing.During thermal annealing, the deposited metal reacts with the siliconforming a metal silicide.

It is to be understood that the present invention will be described interms of a given illustrative architecture having a wafer; however,other architectures, structures, substrate materials and processfeatures and steps may be varied within the scope of the presentinvention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for an integrated circuit chip may be created in a graphicalcomputer programming language, and stored in a computer storage medium(such as a disk, tape, physical hard drive, or virtual hard drive suchas in a storage access network). If the designer does not fabricatechips or the photolithographic masks used to fabricate chips, thedesigner may transmit the resulting design by physical means (e.g., byproviding a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design is then converted into the appropriateformat (e.g., GDSII) for the fabrication of photolithographic masks,which typically include multiple copies of the chip design in questionthat are to be formed on a wafer. The photolithographic masks areutilized to define areas of the wafer (and/or the layers thereon) to beetched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to FIG. 16, a method of forming an SET is shown. Block1602 forms active layer 206 between sacrificial layers 204 and 208. Asstated above, it is specifically contemplated that active layer 206 maybe formed from doped silicon, while sacrificial layers 204 and 208 maybe formed from silicon germanium. Block 1604 forms a fin 302 in thestack of layers. It should be noted that, although only one fin 302 isdescribed herein for the sake of simplicity, any number of fins can beformed in parallel or in other arrangements.

Block 1606 forms a dummy gate 406 over the fin 302. As stated above, itis specifically contemplated that the dummy gate may be formed fromamorphous or polycrystalline silicon. Block 1607 then forms spacers 404around the dummy gate using, e.g., silicon nitride or some otherhardmask material. The spacer material may be formed conformally andthen etched down using, e.g., a timed anisotropic etch to remove spacermaterial from the horizontal surfaces. Block 1608 forms a dielectriclayer over and around the fins 302 and the dummy gate 406 and block 1610polishes the dielectric layer down to the level of the dummy gate 406using, e.g., CMP.

Block 1612 etches away the dummy gate 406 using, e.g., a dry or wet etchthat selectively removes the dummy gate material, exposing the fin 302.Block 1614 removes exposed sacrificial material to expose the activelayer 804, which is suspended from both ends in the gap 602. Block 1616then forms a gate dielectric 1002 over all surfaces in the gap 602 andblock 1618 forms gate 1004. It is specifically contemplated that thegate dielectric 1002 may be formed from a high-k dielectric material,while the gate 1004 may be formed from a conductive metal.

Block 1620 removes material not covered by the spacers 404, the gatedielectric 1002, and the gate 1004. This may be accomplished using ananisotropic etch such as RIE. Block 1622 uses a lateral etch such as,e.g., a wet chemical etch to remove material from the remaining activelayer to form a quantum dot 1302. Block 1624 then oxidizes the quantumdot 1302 to form tunnel barriers 1402 on the exposed surfaces. Block1626 deposits source and drain regions in contact with the tunnelbarriers 1402 to complete the SET.

Having described preferred embodiments of single-electron transistorwith wrap around-gate (which are intended to be illustrative and notlimiting), it is noted that modifications and variations can be made bypersons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

The invention claimed is:
 1. A method for forming a transistor,comprising: forming a fin comprising an active layer between twosacrificial layers; forming a dummy gate over the fin; forming spacersaround the dummy gate; etching away the dummy gate to form a gap overthe fin; etching away material from the two sacrificial layers in thegap; forming a gate stack around the active layer in the gap; andforming source and drain regions in contact with the active layer. 2.The method of claim 1, further comprising etching the active layer afterforming the gate stack to form a quantum dot.
 3. The method of claim 2,further comprising oxidizing exposed surfaces of the quantum dot to formtunnel barriers, wherein the source and drain regions are in contactwith the tunnel barriers.
 4. The method of claim 2, further comprisingetching away material from the fin that is not covered by the gate andspacers.
 5. The method of claim 4, wherein etching the active layerfurther comprises etching away any remaining sacrificial material. 6.The method of claim 1, further comprising forming a dielectric layerover and around the fin and the gate.
 7. The method of claim 6, furthercomprising polishing the dielectric layer down to a top level of thegate.
 8. The method of claim 1, wherein forming the gate stack aroundthe active layer comprises conformally forming a gate dielectric layeron surfaces in the gap.
 9. The method of claim 8, wherein forming thegate stack around the active layer comprises forming a full wrap-aroundgate around the active layer.
 10. A transistor, comprising: a tunnelbarrier structure; a gate stack that wraps fully around the tunnelbarrier structure, wherein a lower portion of the gate stack has a widthgreater than an upper portion of the gate stack; a conductive source anddrain region, in contact with opposite sides of the tunnel barrierstructure, that pass through openings in respective sides of the gatestack.
 11. The transistor of claim 10, wherein the tunnel barrierstructure is a conductive quantum dot having insulating tunnel barriersat opposite sides.
 12. The transistor of claim 11, wherein theconductive quantum dot comprises doped silicon and the insulating tunnelbarriers comprise oxidized silicon.
 13. The transistor of claim 10,further comprising spacers around an upper portion of the gate stack.14. A transistor, comprising: a tunnel barrier structure; a gate stackthat wraps fully around the tunnel barrier structure, wherein the tunnelbarrier structure has a smaller width than a width of the gate stack; aconductive source and drain region, in contact with opposite sides ofthe tunnel barrier structure, that pass through openings in respectivesides of the gate stack.
 15. The transistor of claim 14, wherein thetunnel barrier structure is a conductive quantum dot having insulatingtunnel barriers at opposite sides.
 16. The transistor of claim 15,wherein the conductive quantum dot comprises doped silicon and theinsulating tunnel barriers comprise oxidized silicon.
 17. The transistorof claim 14, further comprising spacers around an upper portion of thegate stack.